1. Technical Background
The present invention relates to medical devices, and more particularly to a retrievable medical filter.
2. Discussion
Some basic types of vascular filters are generally known, wherein a single filter element, mesh or member extends across the direction of flow inside a blood vessel. Several features may be desirable for vascular filters, including non-surgical or “percutaneous” delivery of the filter to a desired site, and expansion from a preferably small initial size to an expanded working size that matches the vascular anatomy at the desired site. Also, a vascular filter should preferably capture a sufficient percentage of thrombus, while allowing blood to flow freely through the filter.
Another desirable feature is a capability to remain reliably in the desired position in a patient's anatomy, referred to as “position retention.” One simple attempt at position retention is to wedge a vascular filter against the blood vessel wall by sizing it with a dimension slightly larger than the inside diameter of the blood vessel. In addition, a vascular filter should preferably have a design whereby the filter is self-centering and stable in the vessel, such that the filter has a tendency not to “tilt”, which might result in less effective capturing of thrombus. Some vascular filters may be used in the vena cava, and may be described in such event as a “vena cava filter.”
A vascular filter may be delivered through a catheter in a compressed shape, the filter tending to resiliently expand within a blood vessel and to retain the desired position and orientation. The vascular filter tends to trap thrombus or particles, and resist their movement further downstream. The filter includes, in a position of use, an outer shape corresponding to the internal diameter of the blood vessel, and one or more filter elements extending across the vessel.
In the temporal sense, there are three types of filters: (i) permanent filters, intended for permanent implantation; (ii) temporary filters, intended for removal within a time period; and (iii) retrievable filters, in which the physician has the option to implant the filter permanently or to remove the filter after some time. In the case of a retrievable filter, the filter may be designed so that the physician can choose whether to retrieve the filter at a later date, after the filter has been in place for a while. This way, the physician can evaluate the performance of the filter and the patient's condition, before deciding whether to retrieve the filter or not.
Regarding retrieval, one factor is “endothelialization” or in-growth of the vessel wall and tissue around the structural members of the filter. In other words, endothelialization is the healing of the vessel inner surface by endothelial cells, and it is desirable to preserve these endothelial cells when removing a retrievable vascular filter. The improved designs of the present invention tend to minimize any impact during retrieval.
Prior vascular filters have a demonstrated track record of filtering clots due to their filter basket design. However, because of the tendency of neointimal tissue to grow over the struts and other filter elements, may cause the filter to become mechanically interlocked with the tissue. When the tissue develops sufficient strength, the filter may no longer be retrievable. For some prior retrievable filters, this may occur somewhere between two and three weeks following implantation.
There are several connection points where a filter may become interlocked with tissue, depending on the size of the vena cava or other body passage in which the device is placed. These include parallel struts between front and back filter baskets, at a shoulder or the filter basket, and at a diamond bifurcation.
According to the principles of the present invention, in order to retain the clot capturing capabilities of the filter baskets, these new filter baskets may be formed by laser-cutting a series of straight, parallel struts, and then shape-setting them in a way so that they closely resemble the filter baskets. As illustrated in the drawings, by forming the baskets from straight, parallel struts, all connections between struts are removed, so that the filter tends not to become interlocked with tissue.
The filter therefore may be retrievable for a longer time or possibly an indefinite period. It may also be desirable to have a filter which consists of two of these filter baskets, one caudal and one cranial, in order to prevent the filter from tilting. Further, in order to facilitate retrievability, it may also be desirable to connect both filter baskets in the middle, and for both filters to be sloped in the same manner as the caudal basket of the filter. In this manner, the outer legs of the filter can act as anchors to prevent migration of the filter.
This filter design provides the demonstrated clot capturing ability of the filter, but the filter elements are shaped from unconnected, straight parallel struts, which allow the filter to be removed from the tissue at a selected time without a tendency for affecting the vessel wall. This is because these struts are not physically connected to each other, and can separate during retrieval.
In addition, this type of filter is compatible with a bi-directional retrieval system. As shown in FIGS. 4 and 5, because the filter struts are not physically attached to each other, as is the case with prior filters, the catheter can be inserted through the windows of the flower basket, without risking entanglement of the catheter. If the retrieval catheter is inserted through one of the windows of the flower basket, the window of the flower basket can be opened, as shown in FIG. 3, to allow the retrieval catheter to slip through.
It is also possible to change the filter basket by adding or subtracting even numbers of struts, so that the number of windows within the flower basket changes from six to five, seven, or some other number. It is also possible to change the direction that the flow baskets face. That is, the cranial basket can be oriented in the same direction as the cranial basket on another filter.
A vascular filter along the lines of the present invention may provide several advantages, including effectively capturing thrombus while allowing blood flow, and resisting endothelialization of the filter. In other words, the filter enables a physician to have a longer time before choosing whether to retrieve a retrievable filter.
A vascular filter may have an initial compressed shape, in which the filter may have essentially a tubular shape, and may be contained in a lumen or passage defined by a catheter.
After a distal tip of the catheter reaches a desired site for treatment, a wire mandrel or other deployment device may be used to push the filter out of the catheter. And when the filter is released from the catheter, it tends to resiliently expand from the initial compressed shape to an expanded shape. When a vascular filter is retrieved from a blood vessel, the entire filter is resiliently compressed to a relatively small diameter, for extraction through a catheter.
The term “filter” will be used interchangeably, to refer to either (i) a combination device including a resilient scaffold structure with a sleeve covering, or (ii) those portion(s) of the scaffold which operate to capture thrombus.
The term “tubular” is used in its broadest sense, to encompass any structure arranged a radial distance around a longitudinal axis. Accordingly, “tubular” includes any structure that (i) is cylindrical or not, such as for example an elliptical or polygonal cross-section, or any other regular or irregular cross-section; (ii) has a different or changing cross-section along its length; (iii) is arranged around a straight, curving, bent or discontinuous longitudinal axis; (iv) has an imperforate surface, or a periodic or other perforate, irregular or gapped surface or cross-section; (v) is spaced uniformly or irregularly, including being spaced varying radial distances from the longitudinal axis; (vi) has any desired combination of length or cross-sectional size.
A vascular filter may include a first and second filter section, arranged on either side of a body section. The body section and the filter sections thus enclose a space. Due to the elongated shape of the vascular filter, and the arranging of the first and second filter sections on either side of the body member, the present filter may have an enhanced filtering effect. In other words, two opportunities have been created for intercepting thrombus moving inside the blood vessel.
A central tubular section tends to resiliently exert slight outward pressure along a large section of contact area on the blood vessel wall. The sleeve distributes this outward pressure to a greater area. Accordingly, the filter tends to exert some small amount of pressure on the internal wall of the blood vessel, and tends to hold itself in place. The vascular filter will consequently tend not to shift position. In addition, because of this elongated shape the vascular filter tends to center itself within the lumen, and not to rotate transversely or tilt over.
In an example, a vascular filter may be formed out of one single piece, which provides advantages including simplicity.
When viewed along the longitudinal axis of the filter, the filter sections may have the shape of a regular polygon, and thus may provide several smaller filtering “cells”. The purpose of these filtering cells is to intercept thrombus moving inside the blood vessel, and the smaller filtering cells tend to capture more thrombus. All the cells may be of the same size, to provide a uniform filtering effect.
The filter sections, as arranged according to an embodiment described above on either side of the tubular body section, may be identical in shape, thereby enhancing the simplicity of the vascular filter according to the present invention.
It is of course possible to build various vascular filters according to the present invention, by various techniques and of various materials to obtain the desired features. It should be noted that the present invention also relates to methods for manufacturing vascular filters, and for using vascular filters for medical treatment of a patient.
These and various other objects, advantages and features of the invention will become apparent from the following description and claims, when considered in conjunction with the appended drawings. The invention will be explained in greater detail below with reference to the attached drawings of a number of examples of embodiments of the present invention.